RF LDMOS devices are commonly used in high-power RF applications, such as RF base stations and RF broadcast stations. In general, RF LDMOS devices are used in plurality to form an array, in which each device is required to have substantial reliability in operation to avoid the whole array becoming useless upon unexpected failure (e.g., premature breakdown) of any susceptible RF LDMOS device.
FIG. 1 shows an existing RF LDMOS device, which can be either a P-channel or N-channel one. In the case of an N-channel RF LDMOS device, as illustrated in FIG. 1, the reference number 1 represents a heavily-doped P-type substrate whereon a lightly-doped P-type epitaxial layer 2 is formed. In the lightly-doped P-type epitaxial layer 2, there are sequentially formed a heavily-doped N-type source region 8, a P-type channel region 7 and an N-type drift region 3 in this order in a side-by-side manner, with a heavily-doped N-type drain region 9 formed in the N-type drift region 3. The P-type channel region 7 and the N-type drift region 3 are overlaid by a gate oxide layer 4 and a uniformly-doped polysilicon gate electrode 5 and stacked in the order from the bottom up. The polysilicon gate electrode 5 and a portion of the N-type drift region 3 are covered by a silicon oxide layer 10, and a portion of the silicon oxide layer 10 is further covered by a gate shield layer 11 which extends above at least a portion of the N-type drift region 3 while being isolated by the silicon oxide layer 10. A sinker region 12 extends downwards from a surface of the source region 8, through the source region 8 and the epitaxial layer 2, into the substrate 1.
With this design, in the event of a high voltage applied on the drain region 9, a snapback effect will be triggered, which is a predominant cause for burn-out of the RF LDMOS device. Accordingly, for the sake of reliability improvement, it is desirable to raise, as high as possible, the threshold of the drain voltage that can trigger the snapback effect. The threshold voltage can be increased by one or a combination of two or more of the following three measures: a) enhancing a breakdown voltage between the drain region 9 and the substrate 1; b) reducing the channel resistance of the device; and c) lowering a channel current upon the breakdown of the device. However, as for measure a), since the breakdown voltage is determined by dopant concentrations of the drain region 9 and the substrate 1, it is not adjustable as desired for a given design with predetermined dopant concentrations of the drain region 9 and the substrate 1.